1. Field of the Invention
The present invention relates to an improved electronic mail (E-mail) program which can protect an existing file filed within a terminal equipment (e.g., a personal computer) or a computer system, from infection by a computer virus, when a user receives an attached file sent from a transmitter through a communication network.
2. Description of the Related Art
In general, there are two representative methods for protecting an existing file of the user from infection by a computer virus.
In a first method, the user confirms whether the received file is infected with a computer virus, using a predetermined virus-checking program which is provided separately from an E-mail program, before a user opens the received file.
In a second method, the E-mail program itself has a function for protecting the received file from the infection by the computer virus. For example, in a certain E-mail program, when a macro process is needed to execute the attached file, the macro process is invalided in accordance with a predetermined security level which is previously set by the user, or an alarm is displayed on a screen. In this case, the security level is defined by three levels, i.e., “high”, “medium” and “low”.
When the security level is “high”, all macros, in which a digital signature by an original designer is not provided, are invalided. When the security level is “medium”, an alarm message is displayed when the macro process is required by the attached file transmitted from a transmitter which has not registered in a list of transmitters. In this case, the transmitters are listed as the transmitters in which the macro process should be permitted. When the security level is “low”, the macro process is always valid. As is obvious from the above, the feature of the second method lies in that an identity of a transmitter is considered and confirmed as a criterion of the above security level.
There is, however, a problem in each of the first and second methods, as explained below.
In the first method, i.e., use of the computer virus check program, a user may accidentally open the attached file without checking of the infection of the computer virus. In general, it is impossible to completely avoid this accidental mistake, i.e., “accidental opening of the attached file without checking the infection of the computer virus”, in a user side.
In the second method, i.e., confirmation of the identity of the transmitter, much handling time is required for utilizing the digital signature. Further, it is very difficult to determine whether the user trusts the transmitter (i.e., whether such transmitter should be registered in the list of transmitter) in a network society, because almost all transmitters utilize anonymity when communicating with one another on the network.
Further, due to the contents of the macro process (for example, whether a file referred to, whether a file is overwritten, and the like), or in the case that the user wishes to determine go/no of execution of the macro process in accordance with a file to be objected by the macro process, it is impossible to avoid the infection by the computer virus even if the second method is used.